1. Field of the Invention
The present invention relates to an electrode unit for in-situ cleaning in a thermal CVD apparatus, and, more particularly, to an electrode unit suitable for in-situ cleaning in a thermal CVD apparatus, improving cleaning rate and productivity in thin film deposition.
2. Description of the Related Art
In recent manufacturing of semiconductor chips, electronic circuit elements in the semiconductor chips are more and more integrated and miniaturized. Miniaturization of the elements in the manufacturing process requires new techniques. These techniques are, for example, of sufficiently filling fine holes with films, forming favorable step coverage, reducing heat due to high current density, and preventing damage to wiring due to electromigration. As one of the manufacturing processes meeting such technical requirements, in place of a sputtering process, a thermal CVD (chemical vapor deposition) process attracts attention. At present, in accordance with the thermal CVD process, blanket tungsten film or TiN film is mainly deposited on a substrate. In the blanket tungsten film deposition H.sub.2 and WF.sub.6 (tungsten hexafluoride) gases are used. The blanket tungsten film is hereinafter referred as a "B-W film". In the TiN film deposition an organic metal gas such as TDEAT (tetradiethylaminotitanium) or TDMAT (tetradimethylaminotitanium) is used as precursors.
When utilizing the thermal CVD process, step coverage can be sufficiently improved for fine holes with a diameter of 0.35 .mu.m or less and an aspect ratio of 4 or more. Thus, the thermal CVD process satisfies the requirements of planarization steps of elements and preventing electro-migration.
A conventional example of a thermal CVD apparatus for depositing the B-W films on a substrate will be described with reference to FIG. 7.
The conventional thermal CVD apparatus shown in FIG. 7 has a gas supply plate 112 at the top of a reactor 111 and a conductive substrate holder 113 at the bottom thereof. A substrate 114 is located on the top surface of the substrate holder 113. The top surface of the substrate holder 113 is circular, and the substrate holder 113 is supported by a plurality of insulating supporting members 115. The reactor 111 has a transmitting quartz window 116 in its bottom wall.
The gas supply plate 112 has a plurality of gas outlets 117 in its bottom surface. Reactive gas supplied by a reactive gas supply mechanism 118 passes through a gas supply pipe 119 and is blown off from the gas outlets 117 so as to be introduced into the reactor 111. The bottom surface of the gas supply plate 112 is opposed to the substrate 114 placed on the substrate holder 113. The reactive gas supplied from the gas supply plate 112 is excited in the space between the gas supply plate 112 and the substrate holder 113. Consequently, a desired thin film can be deposited on the top surface of the substrate 114 placed on the substrate holder 113. Unreacted gas and by-products generated within the reactor 111 are evacuated to the outside through an evacuation section 120. The illustration of an evacuation vacuum pump is omitted in FIG. 7.
A shield member 121 is arranged around the substrate holder 113. This shield member 121 has a ring plate portion 121a and a cylindrical portion 121b. The shield member 121 is conductive. The ring plate portion 121a is located above the substrate holder 113 and around the substrate 114. In accordance with their positional relationship, there is a gap between the ring plate portion 121a and the top surface of the substrate holder 113. The cylindrical portion 121b is also located around the substrate holder 113. Further, the shield member 121 is located on a ring-shaped insulating member 122 disposed on the bottom wall of the reactor 111.
A combination of the bottom wall of the reactor 111, the ring plate portion 121a, the cylindrical portion 121b, and the substrate holder 113 forms a path through which a purge gas (an inert gas) flows. The purge gas is supplied by a purge gas supply mechanism 123 through a purge gas introduction section 124. The purge gas introduced into the reactor 111 via the purge gas introduction section 124 passes through the gap between the ring plate portion 121a and the substrate holder 113. After the purge gas passes through the gap, it is blown off into the reactor 111 from the overall periphery of the substrate 114. This configuration prevents the reactive gas supplied from the gas supply plate 112 from entering the gap. Therefore, the purge gas can prevent unwanted films from being deposited around the circumference of the substrate holder 113 or on the quartz window 116. Further, the shield member 121 serves to prevent the heated substrate holder 113 from being directly exposed to the reactive gas.
An annular lamp support member 125 with a reflecting section is arranged below the quartz window 116 in the bottom wall of the reactor 111. A plurality of heating lamps 126 are mounted on the lamp support member 125 at an almost equal interval. Radiant heat from the heating lamps 126 is transmitted to the substrate holder 113 through the quartz window 116 to heat the substrate holder 113. The substrate 114 is also heated by heat transferred from the substrate holder 113. The temperature of the substrate holder 113 is detected by a thermocouple 127 embedded therein, and a detected signal is fed back to a controller (not shown) so as to be used to control the temperature of the substrate holder 113.
The reactor 111 is electrically grounded at a ground point 128. In addition, a high frequency power supply 130 is connected to both the substrate holder 113 and the shielding member 121 via a matching circuit 129. High frequency power is supplied to both the substrate holder 113 and the shielding member 121. Reference numeral 131 designates an insulating section of the reactor 111 into which a connection line for the high frequency power is inserted.
In accordance with the above configuration, the reactive gas is supplied to the substrate 114 placed on the substrate holder 113, and the reactive gas is excited in the space between the gas supply plate 112 and the substrate holder 113 or the shield member 121, thereby depositing the desired thin film on the substrate 114. The unreacted gas and by-products in the reactor 111 are evacuated by the evacuation section 120. During a thin film deposition process, the purge gas supplied by the purge gas introduction section 124 via the purge gas supply path is continued to be blown off from the above-mentioned gap. The blow-off of the purge gas from the periphery of the substrate 114 prevents the reactive gas from entering the gap, thereby preventing the unwanted films from being deposited on the quartz window 116 and the like.
The normal conditions for the thin film deposition in the conventional thermal CVD apparatus are as follows. In the initial stage of nucleation, the conditions are a 2-10 sccm flow rate for WF.sub.6 as the reactive gas, a 2-10 sccm flow rate for SiH.sub.4, a 100-500 sccm flow rate for Ar as the purge gas, 400-500.degree. C. for the temperature, and 66-1330 Pa for the pressure. In the subsequent stage of film growth by H.sub.2 reduction, the conditions are a 50-200 sccm flow rate for WF.sub.6, a 500-2000 sccm flow rate for H.sub.2, a 300-1000 sccm flow rate for Ar as the purge gas, 400-500.degree. C. for the temperature, and 4000-9000 Pa for the pressure.
On the other hand, as shown in FIG. 8, films 132 as deposits are deposited on the part of the ring plate portion 121a, or the part of the substrate holder 113 which is not covered by the substrate 114. Of course, the deposited films 132 are not desirable because they generate contamination particles and therefore productivity in the thin film deposition process may be reduced. Accordingly, in the conventional thermal CVD apparatus, a RIE (reactive ion etching) process was carried out in order to remove the undesirable deposited films 132 as an in-situ cleaning process in the reactor 111 for each substrate or every certain number of lots. The cleaning process by the RIE was carried out by an cleaning gas. This cleaning gas is supplied by a cleaning gas supply mechanism 133 via the gas supply pipe 119, and is introduced into the reactor 111 through the gas supply plate 112. Further, plasma discharge with the cleaning gas is generated.
If CF.sub.4 and O.sub.2 are used as the cleaning gas, the normal cleaning conditions are a 50-150 sccm flow rate for CF.sub.4, a 20-90 sccm flow rate for O.sub.2, 90-300 Pa for the pressure, and 300-600 W for the high frequency power applied to the substrate holder 113 and the shield member 121.
As being clear in accordance with the above description, the productivity of the conventional thermal CVD apparatus depends on both the deposition rate in the film deposition process and the cleaning rate in the cleaning process.
The above-mentioned conventional thermal CVD apparatus has the following problems.
In accordance with the thermal CVD apparatus having a parallel plate electrode unit, if the RIE discharge process is carried out in order to clean the unwanted films 132 deposited the top surface of the subtract holder 113 or the shield member 121, the cleaning discharge due to the RIE occurs all over the space between the substrate holder 113 and the gas supply plate 112. The RIE discharge process causes a wide discharge in an area wider than the area necessary for the unwanted films 132 to be removed, thereby allowing the power of the discharge for RIE to be dispersed and furthermore the cleaning rate for removing the films 132 to be reduced.
In addition, in the conventional thermal CVD apparatus, if the pressure for the RIE discharge is increased to improve the cleaning rate, the RIE discharge becomes unstable. Further the RIE discharge is apt to occur in a relatively narrow space, for example, in the space between the shield member 121 and the wall of the reactor 111 rather than the space between the gas supply plate 112 and the substrate holer 113. Consequently, the high pressure results in low efficiency.
Even if the pressure for the RIE discharge is relatively high, the RIE discharge may be stabilized by reducing the distance between the gas supply plate 112 and the substrate holder 113. The substrate holder 113, however, is normally maintained at high temperature, that is, 400-500.degree. C., so the temperature of the surface close to the substrate holder 113 increases, and the film may be deposited on the surface of the gas supply plate 112. The unwanted films will generate contamination particles that may reduce the productivity in the thermal CVD apparatus. In addition, a special drive mechanism for vertically moving the gas supply plate 112 or substrate holder 113 will be required in order to have a space for transferring substrates. Consequently, the internal structure of the thermal CVD apparatus is complicated, thereby decreasing the maintainability and increasing the trouble rate.